Many electronic devices, such as personal computers, workstations, computer servers, mainframes and other computer related equipment such as printers, scanners and hard disk drives use memory devices that provide substantial data storage capability, while incurring low power consumption. One type of memory device that is well-suited for use in such devices is dynamic random access memory (DRAM).
Generally, DRAM includes a memory array having a plurality of memory cells that can be arranged in rows and columns. Conductive word lines may be positioned along the rows of the array to couple cells in respective rows, while conductive bit lines may be positioned along columns of the array and coupled to cells in the respective columns. The memory cells in the array may include an access device, such as a transistor device, and a storage device, such as a capacitor. The access device and the storage device may be coupled so that information is stored within a memory cell by imposing a predetermined charge state (corresponding to a selected logic level) on the storage device, and retrieved by accessing the charge state through the access device. Since the charge state in the storage device typically dissipates due to leakage from the cell, the storage device within each memory cell may be periodically refreshed. Current leakage from the cells in the DRAM may occur along several different paths, and if the current leakage is excessive, then the cell refresh interval may be relatively short, which can adversely affect access time for the memory device, and increase the amount of power consumed.
As the cell density of memory devices increases, semiconductor devices, such as access devices, that are vertically disposed in a supporting substrate are increasingly favored. Although a vertical semiconductor device has a reduced footprint when compared to a laterally-disposed device, electrical device isolation presents a concern with ever increasing packing densities.